Answer to Question #8505 Submitted to "Ask the Experts"

From International Commission on Radiological Protection (ICRP) issue 74 (1996) I found that air kerma rate is converted to tissue equivalent dose using a factor of Sv/Gy = 1.21 for ¹³⁷Cs at 0.662 MeV. If I measure with an instrument in nSv, say 70 nSv h^-1, and I want to convert it to Gy, what do I have to do? Do I multiply 70 nSv h^-1 with 1.21 or divide 70 nSv h^-1 with 1.21. Also, what is the factor that we can use for naturally occurring radioactive material (NORM) to convert from Sv to Gy? If some instrument is calibrated using a NORM source and then if you want to recalibrate the same instrument with ¹³⁷Cs, what correction factor do we have to apply?

You can tell from dimensional analysis how the conversion factor must be applied to make the conversion from Sv to Gy. If the air kerma-to-ambient dose equivalent conversion factor given in ICRP is 1.21 Sv Gy^-1, then to convert a reading in nSv to nGy (air) you must divide the reading by the conversion factor—i.e., 70 nSv/1.21 nSv/nGy = 58 nGy. The factor you would use for NORM would depend on the energy characteristics of the gamma radiation emitted by the NORM, which in turn depends on what radionuclides are present.

For NORM the range of energies can be rather large, and different dose-conversion factors may apply at different energies, in which case you might have to generate an effective value weighted according to the gamma yields at the various energies. For ²²⁶Ra, with all the progeny through ²¹⁰Bi in equilibrium, the weighted value of the ambient dose equivalent per unit air kerma is about 1.20, not significantly different from the value for ¹³⁷Cs.

If you calibrated an instrument for NORM and wanted to recalibrate the instrument for ¹³⁷Cs, you would simply calculate the expected value of the ambient dose equivalent rate at the calibration location, just as you would in any other case, and adjust the instrument reading to the expected value. If your question was different and you wanted to know, once having calibrated the instrument for NORM, how you would then measure ambient dose equivalent in a ¹³⁷Cs field, the process would depend on the instrument response characteristics.

If the instrument has been calibrated to read ambient dose equivalent, you must know its energy-response characteristics at the energies of interest. If the energy-response curve shows that the ambient dose equivalent response is acceptably flat (no appreciable energy dependence over the energy range of interest) then no correction would be necessary. Otherwise you may have to adjust the reading according to the energy dependence, and the ICRP conversion factors may not play any direct role.

If you were using an instrument that measured air kerma, and you calibrated it so that the numerical readout was equal to the expected ambient dose equivalent for NORM, and then placed the instrument in a ¹³⁷Cs field, then the reading would have to be multiplied by the quotient of the ¹³⁷Cs ICRP conversion factor by the NORM ICRP conversion factor to obtain the ambient dose equivalent. Good luck.

George Chabot, PhD, CHP

Reference
International Commission on Radiological Protection. Conversion coefficients for use in radiological protection against external radiation. New York: Pergamon Press; ICRP Publication 74; 1996.